Mark TestCaseUnion as a mixin
Mark TestCaseUnion as a mixin

Summary: Now that unions work as mixins

Reviewed By: Mizuchi

Differential Revision: D24739959

fbshipit-source-id: fba82b6a7abdb9c432c1901adfd28f49e0367bcb

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift